Infrared predissociation of ternary cluster cations: the solvent effects on the branching ratio

Branching ratio in photodissociation of 1-bromo-3-chlorobenzene cation

A new and convenient calculation method based on Rice-Ramsperger-Kassel-Marcus (RRKM) theory assuming an extremely loose transition state (LTS) has been attempted to predict the branching ratio in photodissociation. This method enables estimation of the branching ratios without detailed structural information on the transition state which is indispensable in conventional RRKM calculations. To evaluate our simple method through comparison to the experimental results, photodissociation of 1-bromo-3-chlorobenzene cation (3BCB⁺) was chosen as a model unimolecular reaction system which has two distinct photodissociation channels in ultraviolet region: 3BCB⁺ → Br-dissociated daughter ion (ClBz⁺) + Br and 3BCB⁺ → Cl-dissociated daughter ion (BrBz⁺) + Cl. The branching ratio was monitored with decreasing the internal energy using a linear tandem time-of-flight mass spectrometer, which clearly showed decreased branching ratios of 3BCB⁺ → ClBz⁺ + Br over 3BCB⁺ → BrBz⁺ + Cl in reasonable agreement with the calculation results employing the new method. Although there was some discrepancy in internal energy between the experimental and calculation results, the new calculation method is worth to be extended to other diverse systems considering its intuitive and simple nature.

Structures and infrared photodissociation of [(aniline)-(methanol)-(water)2]. SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2019;220:117119. [PMID: 31141781 DOI: 10.1016/j.saa.2019.05.024] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The structures of [(aniline)-(methanol)-(water)₂]⁺ were investigated by infrared spectroscopy coupled with linear tandem mass spectrometry. We suggest the most stable structure of [(aniline)-(methanol)-(water)₂]⁺ through infrared photodissociation spectra supported by the density functional theory calculations at the level of ωB97X-D/cc-pVQZ. Methanol and one water molecule formed hydrogen bonding with the amino group of aniline, while the other water formed hydrogen bonding with methanol. Upon infrared excitation of [(aniline)-(methanol)-(water)₂]⁺, the water molecule connected to methanol turned out to be preferentially ejected, although the total internal energy in the cluster ion was large enough to dissociate other solvent molecules. This unique dissociation feature was attributed to the significant difference in the dissociation rates as obtained by the Rice-Ramsperger-Kassel-Marcus theory calculations as well as structural restriction.

Structures of aniline(pyrrole)+, aniline(ethanol)+, and aniline-(benzene). SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2018;204:665-669. [PMID: 29982157 DOI: 10.1016/j.saa.2018.07.002] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Molecular structures of aniline(pyrrole)⁺, aniline(ethanol)⁺, and aniline(benzene)⁺ produced via resonance two-photon ionization at 266 nm were analyzed by infrared predissociation spectroscopy coupled with tandem mass spectrometry. Structural optimization and frequency calculation using density functional theory were carried out to suggest the most probable isomers which are in good agreement with the observed infrared absorption spectra. Intermolecular bonds in the cluster ions were formed such that the electronegative oxygen atom of the solvent molecule or the pi electron of the aromatic ring forms a hydrogen bonding to NH of aniline.